1. Field of the Invention
The present invention relates to improvements in an electric power steering apparatus for applying power from an electric motor to the steering system of a vehicle to lighten the manual steering effort of the driver, and more particularly to an electric power steering apparatus having a variable ratio steering gear mechanism.
2. Description of the Related Art
Electric power steering apparatuses with a variable ratio steering gear mechanism are known in which the steering gear ratio is designed to gradually decrease with an angle of rotation of the steering wheel in a direction from the straight ahead (neutral) position toward a left or a right end (steering lock) position of the steering wheel. The variable steering gear ratio provides a small lock-to-lock steering angle and prevents excessively high sensitivity or response to the driver""s manual steering operation while the vehicle is running straight. An electric motor is assembled with the variable ratio steering gear mechanism to supply the latter with an assist torque corresponding to the steering torque so as not to increase the steering torque.
Some known electric power steering apparatuses have a gear-ratio characteristic which is responsive to the vehicle velocity. In this case, the steering gear ratio gradually decreases with an angle of rotation of the steering wheel from the straight ahead position toward the left or right steering lock position.
Because of the steering gear ratio gradually decreasing with an increase in the angle of rotation of the steering wheel, application of the steering assist torque from the electric motor in correspondence to the steering torque or in response to the vehicle velocity will cause the problem as described below.
The electric power steering apparatuses with variable ratio steering gear mechanism, when the steering wheel is steered or turned at the same speed as conventional electric steering apparatus with a constant ratio steering gear mechanism, tends to cause over-steering of the front wheels because the steering torque (steering counter force) does not increase so much as the steering angle increases. If a quick steering operation is taken by the driver to correct the oversteer, smooth maneuvers of the vehicle cannot be achieved.
The foregoing problem may be overcome by controlling the apparatus in such a way to apply a steering counter force in response to the steering speed, however, such control will hinder high-speed handling of the steering wheel.
It is accordingly an object of the present invention to provide an electric power steering apparatus which is able to diminish the difference or lag between the driver""s steering operation and the vehicle behavioral response, insures smooth maneuvers of the vehicle, and can apply a steering counter force which enables the driver to steer or turn the steering wheel at high speeds without causing oversteer of the front wheels.
According to the present invention, there is provided an electric power steering apparatus including a variable ratio steering gear mechanism, comprising: an electric motor for applying a steering assist torque to a steering system of the vehicle; a steering torque sensor for detecting a steering torque of the steering system and generating a steering torque signal corresponding to the detected steering torque; a behavior sensor for detecting a behavior of the vehicle and generating a behavior signal corresponding to the detected vehicle behavior; a control unit for controlling operation of the electric motor on the basis of at least the steering torque, the control unit including a steering torque calculating part for outputting a steering torque component corresponding to the steering torque signal output from the steering torque sensor, and a vehicle behavior calculating part for outputting a behavior component corresponding to the behavior signal output from the behavior sensor, the control unit correcting the steering torque component by damping it with the behavior component; and a motor drive unit for driving the motor on the basis of an output signal from the control unit.
Even when the steering wheel is turned at the same speed as the conventional electric power steering apparatus, the vehicle behavior available for the driver as a steering counter force prevents front wheels of the vehicle from becoming over-steered and can diminish the difference or lag between the driver""s manual steering operation and the vehicle behavioral response.
The control unit may further include a vehicle behavioral change rate calculating part for outputting a change rate component corresponding to the rate of change of an output signal from the vehicle behavior calculating part. The control unit further corrects the steering torque component by subtracting the change rate component from the steering torque component.
In one preferred form, the electric power steering apparatus further includes a vehicle velocity sensor for detecting a velocity of the vehicle and generating a vehicle velocity signal corresponding to the detected velocity of the vehicle. The control unit further includes a vehicle velocity calculating part for outputting a vehicle velocity component corresponding to the vehicle velocity signal output from the vehicle velocity sensor, and a steering condition detecting section for detecting a forward state and a return state of the steering system. The control unit corrects the steering torque component by subtracting from the steering torque component a first value determined on the basis of the behavior component and the vehicle velocity component when the forward state is detected by the steering condition detecting section and by adding the steering torque component and a second value determined on the basis of the behavior component and the vehicle velocity component when the return state is detected by the steering condition detecting section.
The steering counter force is well adapted to the sensitivity of the vehicle behavior which is variable with the vehicle speed (the sensitivity is low at low vehicle speeds and high at high vehicle speeds). Accordingly, the steering operation is achieved smoothly throughout the prescribed speed range without creating a great lag between the driver""s manual steering effort and the vehicle behavioral response.
In another preferred form, the electric power steering apparatus further includes a vehicle velocity sensor for detecting a velocity of the vehicle and generating a vehicle velocity signal corresponding to the detected velocity of the vehicle, and the control unit further includes a vehicle behavioral change rate calculating part for outputting a change rate component corresponding to the rate of change of an output signal from the vehicle behavior calculating part, a vehicle velocity calculating part for outputting a vehicle velocity component corresponding to the vehicle velocity signal output from the vehicle velocity sensor, and a steering condition detecting section for detecting a forward state and a return state of the steering system. The control unit, when the forward state is detected by the steering condition detecting section, corrects the steering torque component by first subtracting from the steering torque component a first value determined on the basis of the behavior component and the vehicle velocity component and further subtracting from the resulting remainder of the first subtraction a second value determined on the basis of the change rate component and the vehicle velocity component. When the return state is detected by the steering condition detecting section, the control unit corrects the steering torque component by adding the steering torque component and a third value determined on the basis of the behavior component and the vehicle velocity component, and further subtracting from the resulting sum of the addition a forth value determined on the basis of the change rate component and the vehicle velocity component. Since the steering counter force is also responsive to the vehicle behavioral change rate, the lag between the driver""s steering operation and the vehicle behavioral response is smoothed.
The above and other objects, features and advantages of the present invention will become apparent to those versed in the art upon making reference to the following description and accompanying sheets of drawings in which certain preferred structural embodiments of the present invention are described by way of illustrative examples.